1. Field of Invention
This invention pertains to a method and apparatus for the purification and reuse or disposal of polluted liquids.
More particularly, this invention pertains to an electrodialysis stack for the removal and concentration of ions from aqueous solutions and certain aqueous/organic solutions.
2. Description of the Related Art
There are presently a number of systems for treating and recycling aqueous and aqueous/organic waste streams on the market. Present state of the art systems, including de-ionization methods that are available to industrial waste stream generators, are deficient in their ability to consistently and economically produce a cleansed fluid of sufficient quality that can be continuously recycled and reused, especially in the case of small to medium volume liquid waste generation. The initial high cost of purchasing many of these systems is beyond the economic resources of many businesses, thus prohibiting cost-effective recycling for environmental compliance or beneficial reuse.
Multi-cell electrodialysis stacks are normally built up of membrane sheets separated from each other by suitable gaskets. For efficient separations, the distance (gap) between the sheets is as small as possible. In most designs, a spacer is introduced between the individual membrane sheets, both to assist in supporting the membrane and to help control the liquid flow distribution. The stacks for most electrodialysis processes are assembled in the same fashion as a plate-and-frame filter press, the gaskets corresponding to the frames and the membrane sheets corresponding to the plates. The manifolds that are needed to distribute the process fluids to the various compartments or channels are formed by ingenious patterns of mating holes and slots punched in the gaskets and sometimes in the membranes themselves, prior to assembly of the stack. Several different gasket and spacer materials and arrangements and channel geometries have been utilized or proposed.
In typical electrodialysis systems, the flow pattern within each compartment (i.e., between any two successive membranes) is determined by the configuration of the spacer element used between the membranes. Two distinctively different flow arrangements are typically used. One is known as the tortuous-path design; the other makes use of the sheet-flow principle. The most serious design problem for both flow arrangements for multi-membrane and multi-cell stacks is that of assuring uniform fluid flow to the various compartments and effective transport of the ions to the membrane surfaces. These difficulties are the major obstacles to simple, single stage demineralization of brackish liquids.
In particular, reducing concentration polarization is one of the most important design issues for electrodialysis. Concentration polarization is the reduction of ion concentrations near the membrane surface compared to those in the bulk solution flowing through the membrane compartment. With substantial concentration polarization, electrolytic water splitting in order to provide the requisite electric current carriers through the membranes occurs due to the deficiency of solute ions adjacent to the membranes that can carry the current. This water splitting is extremely detrimental to electrodialysis efficiency. The tendency of concentration polarization to take place at the surface of the membranes is due to the hydrodynamic characteristic of channel flow, in which there is a central turbulent core of flow bounded by thin viscous boundary layers adjacent to the confining surfaces. These viscous boundary layers impose a resistance to the passage of ions much greater than that of a layer of like thickness in the turbulent core, and hence increase the likelihood of polarization at the membrane surfaces. Polarization is objectionable not only from the standpoint of the inefficient increase in energy consumption, but also the change of pH of the concentrate stream as a result of water splitting, which tends to cause scale deposition.
When dealing with fluids with very low total dissolved solids (TDS), back diffusion can take place. Back diffusion occurs when the ion concentration in the concentrate stream is substantially higher than the ion concentration in the de-mineralized stream. The result is that some of the ions from the concentrate stream diffuse back through the membrane, against the force of the DC potential, into the de-mineralized stream.
The number of cells in a stack is limited mainly by the practical considerations of assembly and maintenance requirements. Since the failure of a single membrane can seriously impair stack performance, the necessity to be able to disassemble and reassemble a stack to replace a membrane, and the necessity to be able to perform this quickly and easily, effectively limits the number of membranes that can be practically utilized in a stack. As a result, it is often desirable to use several smaller modular-size stacks rather than one large one. This problem has been attacked by using several small subassemblies or packs containing about 50 to 100 cell pairs (CP), and arranging as many as 10 of these packs in series in a single clamping press. A single set of electrodes may be used for the entire assembly (stack) or several electrodes may be used to provide electric staging. However, use of single electrodes for larger assemblies typically causes end-cell heating that results in rapid membrane deterioration.
The present invention serves to expand the possible applications of electrodialysis in that it represents an efficient, small scale electrodialysis system with a configuration allowing cost-effective small-scale applications, while making the large scale applications even more cost-competitive than they currently are.
In accordance with the present invention, a unique gasket design reduces hydraulic pressure drop across the cell stack assembly by eliminating narrow inlet/outlet manifold cutouts inherent with conventional designs. The reduction of hydraulic pressure permits the use of higher flow rates that further reduce concentration polarization, as well as thinner membranes, resulting in improved desalting efficiency, especially for sparingly conductive solutions, and also less sensitivity to the presence of suspended matter.
The novel multiple split cell design can be operated in parallel as a roughing de-mineralizer (or operated in a batch recirculation mode) or operated in series allowing for single-pass continuous flow. When operated in the series mode, the split cell design permits separate voltage and flow control when a higher purity fluid is desired. The split cell design permits separate cell control of concentrate stream salinity content. The roughing cell may be operated with a higher concentrate stream TDS, with the salinity of the polish cell concentrate stream correspondingly reduced to the salinity content of the de-mineralized stream. This prevents back diffusion and allows for efficient removal of ions in feed water of low TDS. In short, the split cell design incorporates the benefits of hydraulic and electrical staging without the inherent complexity and expense of commercial electrodialysis systems.
The split cell design minimizes the voltage potential across the stack, thereby reducing end-cell heating that leads to membrane deterioration.
It is an object of the present invention to provide a simpler stack assembly of low production cost. Stack assembly cost is reduced as a result of the novel split cell/gasket geometry. A reduced number of expensive machined components are required. Simpler and lighter components lower material costs for a given membrane area. Inexpensive center bolts provide an alternative to typical hydraulic force application arrangements, which also improves the uniformity of the clamping force distribution on the gasket area. Threaded bolts also reduce assembly labor time, i.e., it is easier to hold the configuration in place and also facilitate change-out of membranes when they are spent, as the cell geometry reduces stress on the end points as is found inherent with some conventional stack assemblies.
It is another object of the present invention to provide an apparatus and method that allows for the cost-effective arrangement of two or more split membrane cells that enables the ingenious arrangements of plumbing for optimizing deionization processing.
The cell gasket geometry can be more easily and inexpensively fabricated from a larger range of materials in comparison to conventional designs, allowing the process to be used in more harsh environments through the use of a wide range of chemically resistant materials. It is still another object of the present invention to provide an apparatus and method that combines a unique arrangement of small to intermediate scale unit operations for the economical recovery/reclamation of a wide range of fluids and that can also be scaled to a large system size, further improving the economics of large scale electrodialysis systems by reducing both capital and operating costs.